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Source:互联网Author:qyadmin Addtime:2019-11-13 16:34 Click:

Backup board for standard pcb drill holes

PCB drill holes sizes Tolerances


      subtract the tolerance from the standard value to find the smallest. and add the tolerance to the standard value to find the largest. Thus, the actual part can vary from 0.019 to 0.025.In practice, you will find most of the parts are close to 0.022. ,you’ll find a part that is outside of the specified range. which would considered a defective part because it didn’t meet specifications.
 
       if the resistor can only be as large as 0.025 inch, why make the hole 0.029? Ahh -- because the hole has tolerances, too!
 
      As each hole drilled, the drill bit wears down a little bit. Or, the drill may wiggle or vibrate in a hole, causing a oversized hole. After that, the hole plated. The plating may be thicker or thinner for each batch or position on the board. You also have to account for thermal expansion or shrinkage of the PCB substrate. as it is being processed.
 
      Many PCB manufacturers specify a hole tolerance of ±0.004. So, 0.022 resistor lead + 0.003 resistor lead tolerance + 0.004 PCB hole tolerance = 0.029 inches.
 
      The 0.007 inch rule of thumb is adequate for hobbyists and prototyping most of the time. Of course. it is better to check out the datasheets and online specifications for the part. and PCB manufacturer you will be using. Often, the specified tolerance will vary based on hole size and type of plating. In fact, professional designers also. take into account the type of solder and the soldering process.

pcb drill holes

Backup board for standard pcb drill holes

 
      The most important DFM considerations for drill holes. are generally PCB drill hole tolerance and PCB drill aspect ratio. When determining these, there is much more to consider than the actual drill hole size.
 
      there are two types of drill holes: non-plated. through holes (NPTHs) and plated through holes (PTHs). NPTHs are holes that do not carry current, and thus do not need conductive plating. Mounting holes are the most common type of NPTH.  PTHs, but, are usually signal carriers or ground returns. They carry current and need conductive plating. PTHs are usually vias between outer and inner layers. inner layers only or surface-to-surface.
 
       There are some general rules that should applied for the NPTHs and PTHs of your design. Not employing the following PCB drill hole rules in your DFM. can impact the turnaround time for your board:
 
Drill Hole Rules
 
Non-Plated Through Hole (NPTH)
 
Minimum finished hole size = 0.006″
 
Minimum edge to edge clearance (from any other surface element) = 0.005″
 
Plated Through Hole (PTH)
 
Minimum finished hole size = 0.006″
 
Minimum annular ring size = 0.004”
 
Minimum edge to edge clearance (from any other surface element) = 0.009″
 
If your board needs to be IPC Class 2, then the annular ring must be at least 0.004”. larger than the drilled hole at every point. For example, if your hole is 0.006”, then the smallest pad size for the drilled hole would be 0.014”.
 
When drilling holes, whether mounting or vias. board fabricators limited by board thickness, as well as the diameter. and depth capabilities of their milling machines, lasers or other equipment. These limitations are generally expressed of the aspect ratio. which expresses the proportionality relationship.  between the hole depth and diameter.

Back-Up Material functional

 
      The material structure provides a means to drill a wide variety of multilayer. PCB constructions with varying surface topographies. It shapes itself to the PCB topography on one side, while staying flat on the other. It provides the support needed for copper burr suppression. To get these different and opposing properties from the material. a composite structure employed.
 
      The composite composed of a core and a surface coating on each side. The choice and formulation of materials is critical. to get the proper combination of properties. As an example. a liquid core cannot. used as this requires lateral displacement for changes in height. Conventional back-up is too hard and non-conforming to shape without damage to the PCB. By using a 3 layer composite between the PCB and a standard back-up. the composite can shape itself to various topographies. that may be present in a multilayer. or rigid flex PCB. This allows the back-up to stay in intimate . contact with the surface copper, reducing burrs. 
 
A three layer structure with two layers of bonding adhesive.
 
1. An acrylic coated paper
 
a. A thin adhesive bonding layer
 
b. A low density paper fiber core
 
c. A thin adhesive bonding layer
 
2. An acrylic coated paper
 

Why wood pcb backup baord can use in drill holes?

 
      In the process of micro-hole drilling of printed circuit boards. selecting suitable cover pads. and drilling with various printed circuit boards. can not only improve the drilling quality, improve the yield, but also prolong the life of the drill bit. and reduce the production cost. so, the development of a new type of cover pad with superior performance. and reasonable cost to meet the needs of high-end PCB drilling. has become an important research goal of the manufacturer of the cover plate.
 
      The PCB drilling pad is almost the same as the PCB. During the drilling process, the cover. and the pad added under the PCB to protect the PCB and improve the hole accuracy. The phenolic resin cover plate used. which can meet the requirements of conventional drilling in a certain degree. For the reason of the easy occurrence of drilling, epoxy fiberglass board. was once used in the PCB industry. but it can not completely solve the problem of drilling. so it can not meet the production requirements. and the cost of epoxy fiberglass board is also It is higher, so its history is not very long and it abandoned.

Tags:
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